A satisfactory glass microelectrode has been constructed to facilitate the measurement of pH in nanoliter amounts of collected tubular fluid. Our studies thus far suggest that proximal convoluted tubules are extremely permeable to CO2 and most likely permeable also to bicarbonate. They are capable of generating an acid pH (6.3-6.4) in the absence of bath and luminal CO2 and bicarbonate. The results of steady state studies in the presence of bicarbonate and CO2 await further improvements in the geometry of the collecting pipet, as well as a definite solution to the flow-diffusion equation.